Interpretive Summary: Potato tubers that are not harvested due to small size, improper digger blade depth, falling off the harvester, etc. can result in serious weed problems in the rotation crops. Carrots and onions are often grown in rotation with potatoes and methods to control potatoes in these crops are lacking. Removing potatoes in the rotation crop can cost over $200 per acre following a mild winter in which tubers are not killed by freezing temperatures. Trials conducted by ARS in 1996 and 2000 identified several promising treatments for volunteer potato control in onions that nearly eliminate potatoes in the onion crop. Effective treatments combine sequential applications of herbicides and cultivation and maintained onion yields equal to hand-weeded checks. In carrots, glyphosate applied prior to carrot emergence controlled early-emerged potato shoots, but tubers continued to send new sprouts. Fumigation also is effective in reducing the volunteer potato problem prior to planting carrots. Fluroxypyr controlled potatoes well, but injured carrots.

Technical Abstract:
Carrots and onions are often rotated with potatoes and tubers that survive winter can sprout in the spring and severely reduce carrot and onion growth and yield. Trials were conducted in 1996 and 2000 at Prosser, WA to evaluate herbicide and cultivation treatments for volunteer potato control in onions. Seventeen herbicide treatments were tested in 1996 and thirteen treatments in 2000. All plots were cultivated 7-10 days after eac POST herbicide application in both years of this study. Ethofumesate (Norton) applied PRE at 0.5 lb ai/acre followed by two applications of ethofumesate plus bromoxynil applied POST reduced potato tuber weight and number of tubers, without reducing onion yield compared to handweeded checks. Two POT applications of fluroxypyr (Starane) alone or with bromoxynil at the 2, 3, and 4-5 leaf stages of onion followed by cultivation reduced volunteer potato tuber weight and number of tubers equal to or more than all other treatments and onion yield was equal to that of hand weeded checks. Fumigation prior to carrot planting, cultivations, and hand weeding are the main methods used to control volunteer potatoes in carrots. Carrots were injured by fluroxypyr applied at two stages of growth. Carrot leaves exposed at the time of herbicide application were twisted downward and carrot roots had more hairs and callus-like bumps on the surface several weeks after application. Carrot yield was reduced by fluroxypyr applied at the 0.25 lb ae/a.